A mortar shell fired in the Syrian Golan Heights landed in the area on the Israeli side on Monday morning, the IDF Spokesman's Office said. The army said the mortar was likely fired in heavy fighting between Syrian army forces and rebels in the area, and was not meant for Israel.

Rebels of al-Qaeda-linked Nusra Front have been battling the Syrian army in the area and have wrested control of the crossing at Quneitra, which is operated by the United Nations.

Persistent gunshots and explosions from mortar shells and other munitions could be heard on the Israeli side of the frontier and combatants could be clearly seen targeting each other with their weapons.

Fighting at the Quneitra border crossing (Photo: AP)

A large Syrian flag that had been flying for days between the Quneitra crossing and the abandoned town was taken down and a United Nations position in the area, thought to be unmanned, was pounded with mortar shells.

It was not clear whether the forces loyal to Syrian President Bashar Assad had managed to retake control of the Quneitra crossing from the rebels of the al-Qaeda-linked Nusra Front.

Syria's state news agency said the military killed "many terrorists" and destroyed a heavy machine gun in the fighting. The government refers to those trying to oust President Assad as terrorists.

The Britain-based Syrian Observatory for Human Rights said the fighting on Monday was focused around the town of Hamidiyeh in Quneitra province.

The Observatory, which says it gathers information from all sides in the conflict, said there were confirmed casualties on both sides.

Speaking to Reuters, Observatory founder Rami Abdelrahman said the Nusra Front's aim appeared to be "to end once and for all the regime's presence in the area and it also appears that the goal is to expel the international observers".

Al-Qaeda flag over UN vehicle

Meanwhile, a picture of a UN vehicle flying an al-Qaeda flag was posted on social media and other websites Sunday, by the al-Nursa Front, a militant Islamist group that led the offensive against Syrian government forces.

At least one tank belonging to the Syrian army loyal to President Bashar Assad was also involved in battles near the border, and rebels could be seen a few meters (yards) away from the frontier fence.

It was not immediately clear who dispatched the unmanned aircraft or the nature of its mission in an area where fighting from Syria's civil war has occasionally spilled over into Israeli territory.

In a statement, the military said the drone was downed by a Patriot missile near the Quneitra crossing between the Israeli Golan Heights and Syria.

A jihadist in Syria next to a captured UN vehicle.

A contingent of 44 UN peacekeepers from Fiji was detained on the Syrian side of the Golan by Islamist militants on Thursday, and the head of the Fijian army said on Sunday negotiations for their release were being pursued.

More than 70 Philippine troops trapped by Islamists in a different area on the Syrian side of the frontier were extracted to safety, the United Nations and authorities in Manila said.